Bifocal optical pickup head

ABSTRACT

A bifocal optical pickup head is disclosed for use in a optical pickup apparatus having a single objective lens, for generating two different focuses (bifocus) to access a compact disc (CD) and a digital versatile disc (DVD), which the thickness is different. When a beam passes through an optical element disclosed in the present invention, numerical aperture is controlled and spherical aberration is reduced by the optical element due to variations in optical path and radius. Therefore, after beams having different numerical apertures are focused by an objective lens, various focuses can be obtained.

BACKGROUND OF THE INVENTION

The present invention relates to a bifocal optical pickup head, and moreparticularly to an optical pickup apparatus used with a single objectivelens for accessing a compact disc (CD) and a digital versatile disc(DVD), which are different in thickness.

A technique of accessing data recorded on a surface of an optical discsuch as CD or DVD by an optical pickup head has been known. In reading,a laser beam is focused on a data storage surface of the optical discand the beam reflected from the optical disc is converted to an electricsignal by a photo detector. In writing, the above procedures areinverted. The specifications of CD and DVD are shown in Table 1.

TABLE 1 Thick- NA (numerical ness (mm) Track pitch (μm) Wavelength (nm)aperture) CD 1.2 1.6   780 0.45 DVD 0.6*2 0.74 635-650 0.6

As know from Table 1, in comparison with CD, DVD has a higher storagedensity. The differences in dimension (especially the storage densitydifference) result in various reading requirements for an optical pickupapparatus. To access data on a data storage surface of a CD or a DVD, anoptical pickup head is a key component in the optical pickup apparatus,and optics is most critical in the optical pickup head.

Therefore, to access both CD and DVD by an optical pickup apparatushaving a single objective lens, it is necessary to design an optics inwhich beam spot size is changed on the basis of two different numericalapertures.

To meet compatible requirement for CD and DVD, it is necessary to accessboth CD and DVD with a single optical pickup head, and how to obtain abifocal optics is a critical technique.

Conventional techniques of accessing both CD and DVD with a bifocaloptical pickup head are listed as follows:

1. Two objective lenses having different focal lengths corresponding toCD and DVD respectively are used, and a driving mechanism is utilized toselect one of the objective lenses. Such a design has disadvantages ofincreased weight of an optical pickup head and increased manufacturingcost.

2. A diffraction element is used to achieve bifocal effect.

3. A holographic optical element is used for bifocus. However, itsuffers from manufacturing difficulty and high manufacturing cost.

4. A NA controller is constructed by a liquid crystal display (LCD)shutter such that an objective lens has two NA values, thereby achievingbifocus. However, the orientation of the polarization of a laser sourceneeds to be accurately aligned, and the LCD should be continuouslypowered.

5. An annular objective lens is used in which beams passing throughcenter of the lens are for CD while beams passing though inner and outerrings thereof are for DVD. A similar technique is disclosed in U.S. Pat.No. 5,665,957 in which a hologram lens having a pattern thereon is usedfor bifocus. However, its manufacturing cost is high and mountingtolerance is low.

6. Two light sources and two lens are used. For example, according toU.S. Pat. No. 5,777,970, two laser sources of different wavelengthscorrespond to respective lenses to provide respective focal lengths andspots of respective diameters. It has disadvantages of highmanufacturing cost and complicated structure.

7. A zooming aperture is utilized in association with a movableobjective lens, as shown in U.S. Pat. No. 5,659,533 and U.S. Pat. No.5,281,797.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a bifocal opticalpickup head having advantages of simple structure, low cost, easyassembly and high tolerance.

According to a bifocal optical pickup head of the present invention, toaccess both CD and DVD, an optical element is used in association with aconventional optical pickup head comprising a laser diode, a beamsplitter (or holographic optical element (HOE)), a collimator lens, anobjective lens and a photo detector, without adding any other opticalelements such as zooming aperture, movable objective lens or dualobjective lenses.

Another object of the present invention is to provide an optical elementadapted to a conventional optical pickup head, holographic opticalpickup head or two-source optical pickup head. The optical elementsplits a single laser beam into two laser beams of different opticalpaths, which are collimated by a collimator lens and then focused by anobjective lens to achieve two focuses.

According to the present invention, an optical element disposed in alight path is disclosed. When a beam passes through the optical element,numerical aperture is controlled and spherical aberration is reduced bythe optical element due to variations in optical path and radius.Therefore, after beams having different numerical apertures are focusedby an objective lens, various focuses can be obtained.

When a monochrome beam undergoes two different optical paths, twofocuses can be obtained. According to the present invention, a cylinderor a circular recess is provided at center of a prior art opticalelement, such as three-beam grating or holographic optical element, tosplit a laser beam into two. An optical path difference is caused by thecylinder or circular recess, and NA of the optical element iscontrolled, such that the beam completely passing through the opticalelement is used for DVD and the beam passing through the cylinder orcircular recess is used for CD.

These and other objects, features and advantages of the presentinvention will become apparent to those skilled in the art uponconsideration of the following description of the preferred embodimentsof the present invention taken in conjunction with the accompanyingdrawings

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an optical element of a first embodiment;

FIG. 2 shows a bifocal optical pickup head of the first embodiment ofthe present invention;

FIG. 3 is a partially enlarged view showing a beam focused on a DVD;

FIG. 4 is a partially enlarged view showing a beam focused on a CD;

FIG. 5 shows an optical element of a second embodiment;

FIG. 6 shows a bifocal optical pickup head of the second embodiment ofthe present invention in which the optical element is used with aconventional optical pickup head;

FIG. 7 shows an optical element of a third embodiment in which theoptical element is provided in a three-beam grating;

FIG. 8 shows a bifocal optical pickup head of the third embodiment ofthe present invention in which the optical element is used with aholographic optical pickup head;

FIG. 9 shows a bifocal optical pickup head of a fourth embodiment of thepresent invention in which the combination of an optical element and athree-beam grating is used with a two-source optical pickup head; and

FIG. 10 shows a bifocal optical pickup head of a fifth embodiment of thepresent invention in which an optical element, a three-beam grating anda holographic optical element are used with a two-source optical pickuphead.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, an optical element 10 having a first refractionsurface 11 and a second refraction surface 12 perpendicular to axis of alaser beam in an optical pickup head. The refraction surfaces 11 and 12are located at different places corresponding to different optical pathsand different radiuses of the laser beam. For example, a protrudedcylinder 130 is formed at a side of a flat transparent body 13 such asglass plate or quartz plate having a high transmittance. Surface A ofthe transparent body 13 at the same side with the cylinder 130 is formedas the first refraction surface 11, while axial end surface B of thecylinder 130 is formed as the second refraction surface 12. The surfaceA and the axial end surface B are concentric such that a laser beam willbe split into two beams of different optical paths when it passesthrough the refraction surfaces 11 and 12. Then the beams are collimatedand pass through an objective lens to form two spots of differentfocuses.

Preferably, the radial section of the cylinder 130 has a shape dependingon sectional shape of the laser beam to enhance the efficiency thereof.The optical element 10 is manufactured by photolithography to performpattern transfer, followed by etching wherein a desired etched depth canbe obtained by controlling the etching time. Of course, injectionmolding or the likes can be employed instead.

FIG. 2 shows a bifocal optical pickup head of a first embodiment of thepresent invention. The bifocal optical pickup head comprises: a laserdiode 20 for generating a laser beam for accessing an optical disc 8 (CDor DVD); an optical element 10 for splitting the laser beam emitted fromthe laser diode 20 into two beams of different optical paths; a beamsplitter cube 30 for separating the two laser beams of different opticalpaths from a reflection beam from the optical disc 8 (CD or DVD); acollimator lens 40 for collimating the beams as a parallel beam; anobjective lens 50 for focusing the parallel beam including the two laserbeams of different optical paths on a data storage surface of theoptical disc 8 (CD or DVD); a photo detector 61 for converting thereflection beam from the optical disc 8 (CD or DVD) into an electricsignal; and a condensing lens 60 for focusing the reflection beam fromthe beam splitter cube 30 on the photo detector 61.

As clear from FIG. 2, the laser beam for accessing the optical disc 8(CD or DVD) is emitted from the laser diode 20 through optical element10 where it is split into two laser beams of different optical paths.Then, these two beams pass through the collimator lens 40 and theobjective lens 50, and are focused on the data storage surface of theoptical disc. These two beams passes through the first refractionsurface 11 and the second refraction surface 12, respectively. One ofthe beams having higher efficiency is used for DVD 81 (referring to FIG.3), while the other which passes through only the cylinder 130 is usedfor CD 82 (referring to FIG. 4). Therefore, by the aid of the opticalelement 10 of the present invention, to access both CD 82 and DVD 81,the collimator lens 40 and the objective lens 50 are employed and noother optical devices such as zooming aperture, movable objective lensor dual objective lenses are necessary.

To align center of the cylinder 130 with the laser beam center caneasily assemble the optical element 10. Further, since the beamreflected from the optical disc surface passes through the condensinglens 60 to the photo detector 61 via the beam splitter cube 30 and doesnot returned to the optical element 10, the assembly tolerance ishigher.

FIG. 5 shows another preferred embodiment of an optical element 10 awherein a cylindrical recess 131 is formed on surface of a flattransparent body 13. Axial end surface C of the cylindrical recess 131acts as second refraction surface 12 like axial end surface B in FIG. 1.

FIG. 6 shows a bifocal optical pickup head of a second embodiment of thepresent invention in which how to use an optical element 10 of thepresent invention with a conventional optical pickup head is disclosed.The bifocal optical pickup head comprises: a laser diode 20 for emittinga laser beam for accessing an optical disc 8 (CD or DVD); a three-beamgrating 70; an optical element 10; a beam splitter 30; a collimator lens40; an objective lens 50; a condensing lens 60; and a photo detector 61.

In this embodiment, the optical element 10 can be formed as shown inFIG. 1 or FIG. 5, and can be directly integrated on back of a three-beamgrating 70 or the like.

A holographic optical pickup head shown in FIG. 8 comprises: a laserdiode 20 for emitting a laser beam for accessing an optical disc 8 (CDor DVD); an optical element 10 as shown in FIG. 1 or 5 for splitting thelaser beam emitted from the laser diode 20 into two beams of differentoptical paths; a holographic optical element (HOE) 80 for separating thetwo laser beams of different optical paths from a reflection beam fromthe optical disc 8 (CD or DVD); a collimator lens 40 for collimating thelaser beams passing through the holographic optical element 80 as aparallel beam; an objective lens 50 for focusing the parallel beamincluding the two laser beams of different optical paths on a datastorage surface of CD 82 or DVD 81; and a photo detector 61 forconverting the reflection beam from the holographic optical element 80into an electric signal.

In FIG. 9, a two-source optical pickup head is shown in which an opticalelement 10 is integrated with a three-beam grating 70 (as shown in FIG.7). As shown, two laser diodes, i.e., first laser diode 20 a and secondlaser diode 20 b, which generate laser beams of different wavelengths,are used to access different optical discs 8 (CD or DVD) respectively.The laser beam emitted from the first laser diode 20 a passes throughthe combination of the optical element 10 and the three-beam grating 70,a collimator lens 40 and an objective lens 50, and then is focused on adata storage surface of the optical disc 8. The laser beam emitted fromthe second laser diode 20 b passes through a first beam splitter cube 30a, the collimator lens 40 and the objective lens 50, and then is focusedon the data storage surface of the optical disc 8. The laser beamreflected from the data storage surface of the optical disc 8 passesthrough a second beam splitter cube 30 b and a condensing lens 60, andthen is focused on a photo detector 61 where the laser beam representinga photo signal is converted into an electric signal.

In the example shown in FIG. 9, if the first laser diode 20 a emits alaser beam having wavelength of 780 nm, the inner portion of the laserbeam is used to access CD, and another laser beam having wavelength of650 nm from the second laser diode 20 b is used to access DVD.Therefore, an optical pickup head capable of selectively accessing CD orDVD by the objective lens 50 is formed. If the first laser diode 20 a isa SHG laser with wavelength of 820 nm, the inner portion of the laserbeam accesses CD while the laser beam of second harmonic with wavelengthof 410 nm accesses DVD.

FIG. 10 shows a preferred embodiment in which a three-beam grating 70,an optical element 10 and a holographic optical element 80 are combinedtogether to form an integrated optical element 90, which is used with atwo-source optical pickup head. In this embodiment, a first laser diode20 a with wavelength of 780 nm and a second laser diode 20 b withwavelength of 650 nm are for CD and DVD, respectively.

A laser beam emitted from the first laser diode 20 a passes through theintegrated optical element 90, a collimator lens 40 and an objectivelens 50, and then is focused on a data storage surface of an opticaldisc 8. The laser beam reflected from the data storage surface of theoptical disc 8 follows the opposite optical path, and is focused by theholographic optical element (HOE) 80 in the integrated optical element90 on a photo detector 61 where the laser beam representing a photosignal is converted into an electric signal. A laser beam emitted fromthe second laser diode 20 b passes through a beam splitter cube 30 a,the collimator lens 40 and the objective lens 50, and then is focused onthe data storage surface of the optical disc 8. The laser beam reflectedfrom the data storage surface of the optical disc 8 follows the oppositeoptical path, and is focused by a holographic optical element (HOE) 80 aon another photo detector 61 a where the laser beam representing a photosignal is converted into an electric signal.

According to the present invention, to access both CD and DVD, it isunnecessary to add any other optical elements such as zooming aperture,movable objective lens or dual objective lenses. The present inventionprovides advantages of simple structure, low cost, easy assembly andhigh tolerance. Besides, the optical element of the present invention isadapted to a conventional optical pickup head, holographic opticalpickup head or two-source optical pickup head.

While the present invention has been described in conjunction withpreferred embodiments thereof, it is evident that many alternatives,modifications and variations will be apparent to those skilled in theart. Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and scopethereof as set forth in the appended claims.

INDEX 10 optical element 10a optical element 11 first refraction surface12 second refraction surface 13 transparent body 130 cylinder 131cylindrical recess 20 laser diode 20a first laser diode 20b second laserdiode 30 beam splitter cube 30a first beam splitter cube 30b second beamsplitter cube 40 collimator lens 50 objective lens 60 condensing lens 61photo detector 61a photo detector 70 three-beam grating 8 optical disc80 holographic optical element 80a holographic optical element 81 DVD 82CD 90 integrated optical element

What is claimed is:
 1. A bifocal optical pickup head for accessing anoptical disc, comprising: a laser diode for generating a laser beam foraccessing said optical disc; an optical element for splitting said laserbeam emitted from said laser diode into two laser beams of differentoptical paths; a beam splitter cube for separating said two laser beamsof different optical paths from a reflection beam from said opticaldisc; a collimator lens for collimating said laser beams passing throughsaid optical element as a parallel beam; an objective lens for focusingsaid parallel beam including said two laser beams of different opticalpaths on a data storage surface of said optical disc; a photo detectorfor converting said reflection beam from said optical disc into anelectric signal; and a condensing lens for focusing said reflection beamfrom said beam splitter cube on said photo detector.
 2. The bifocaloptical pickup head according to claim 1, wherein said optical elementcomprises a first and a second refraction surfaces perpendicular toaxial direction of said laser beam, and said first and second refractionsurfaces are concentric about axis of said laser beam and located at twodifferent places corresponding to different optical paths and differentradiuses of said laser beam.
 3. The bifocal optical pickup headaccording to claim 2, wherein said optical element comprises a flattransparent body provided with a protruded cylinder at a side thereof,and a surface of the transparent body at the same side with saidcylinder is formed as said first refraction surface, while an axial endsurface of said cylinder is formed as said second refraction surface. 4.The bifocal optical pickup head according to claim 2, wherein saidoptical element comprises a flat transparent body provided with acylindrical recess at a side thereof, and a surface of the transparentbody at the same side with said cylindrical recess is formed as saidfirst refraction surface, while an axial end surface of said cylindricalrecess is formed as said second refraction surface.